Human babesiosis is a malaria-like illness caused by infection of red blood cells by various species of protozoan parasites of the genus Babesia. B. microti is responsible for most human infections reported in the United States, and this parasite is endemic in parts of the Northeast and upper Midwest. B. microti infections are asymptomatic in most individuals but can lead to severe illness or death, especially in immunosuppressed, asplenic or elderly individuals. The parasite is primarily transmitted to humans by exposure to deer ticks in endemic areas. However, babesiosis can also be transmitted by blood transfusion. The parasite is able to tolerate standard blood banking conditions and adjunct processing procedures (Leiby D A. Vox Sang 2006, 90: 157-165) as evidenced by transmission following transfusion of a whole blood, liquid-stored and frozen-deglycerolized red cells, as well as whole blood derived platelets (Pantanowitz L, et al. Transfusion 2001, 41: 440). Over 100 cases of transfusion-transmitted babesiosis (TTB) have been reported to the FDA since 1979 and at least 12 fatalities since 2005 (Gubernot D M, et al. Clin Infect Dis 2009, 48: 25-30). Despite being acknowledged as the foremost unaddressed infectious risk to the U.S. blood supply, there is an urgent and unmet need for blood donor screening to be implemented (Herwaldt B L, et al. Ann Intern Med 2011, September, 2011:155 (5)). Furthermore, with no assay licensed for blood screening and no donor screening strategy in widespread use, TTB continues to pose risk to transfusion recipients in the US.
Blood donations from Babesia-endemic areas in the U.S. can exhibit up to 1% seropositivity for B. microti, and the distribution of Babesia seropositivity is spreading rapidly to adjacent states and elsewhere. Indeed, the B. microti species of Babesia parasite, hitherto rare in Europe, has been reported in localized regions of Germany and Switzerland, with human seroprevalence between 1% and 9% (Leiby D L et al. Clin Micro Reviews 2011, Jan., 24 (1): 14-28).
Babesiosis has a wide spectrum of clinical presentation that is largely governed by immune status of the host. While the majority of naturally acquired infections are either asymptomatic or mild and self-limiting in the immunocompetent host, infection at extremes of age, in the immunocompromised and/or in individuals with asplenia has a high risk of severe complicated and even fatal disease (Asad S, et al. Transfusion 2009, 49:12, 2564-2573). Transfusion recipients share notable overlap with these high-risk groups and TTB confers an estimated mortality of 5-9%.
The emerging threat of human babesiosis as a transfusion-transmitted disease has led to a consensus by the Food and Drug Administration (FDA) and the American Association of Blood Banks (AABB) that screening of blood donations for Babesia is urgently required for blood safety (Leiby D L, Annals Int Med 2011, August, 155). Recent reports showing an alarming increase in transfusion-transmitted babesiosis (TTB), which can cause serious illness or death in immunocompromised patients, underscore this need. Therefore a quick, easy, and sensitive assay for blood screening and detecting Babesia is needed, and as such would facilitate a ready supply of blood that are free of Babesia contamination and safe for use by humans.
Currently no commercial, validated and FDA approved test is available for this pathogen. B. microti is currently detectable by four principal assay methods: microscopic observation of Giemsa-stained blood smears, indirect immunofluorescence (IFA), PCR or xenodiagnosis by hamster inoculation. The assay performances reported for these methods are compared in Table 1. The IFA method, originally described over 30 years ago (Chisholm E S, et al, Am. J. Trop. Med. Hyg. 1978; 27:14-19; Krause P J, et al. Antibody, J. Infect. Dis. 1994; 169:923-926), remains the only currently available serological method. IFA requires microscopy skills, specific training and access to a fluorescence microscope, which is practical for some reference laboratories, but not a technique amenable to routine use and practice by non-specialists. Examination of thin blood smears for piroplasms similarly requires a microscope and skilled operator and is subject to the same limitations (Leiby D A. Vox Sang 2006, 90: 157-165.). PCR demands a highly controlled environment to avoid contamination and artifactual results, complex and expensive instrumentation and reagents, and a high degree of training to perform properly.    Table 1: Comparative performances between Babesia current tests from Chisholm E S et al. Am. J. Trop. Med. Hyg. 1978; 27:14-19 and Krause P J et al. Antibody, J. Infect. Dis. 1994; 169:923-926.
TABLE 1SensitivitySpecificityPPVNPVBlood smear84%100%100%62%IFA88%-96%90%-100%69%-100%96%-99%PCR95%100%100%83%Xenodiagnosis74%100%100%50%